This article makes use of modulation technic of laser speckle to realize differentiation of the image so as to increase the detail and contour of the image. This kind of technic has many advantages，for example，it has a large field of vision，it is easy to master and by using it，we can get a good result. In achieving differentiation of the image，the article tells us how to use the methods of exposure with different exposure time interval for many times from theory and by giving some results of the experiment.